1. Field of the Invention
The present invention relates to a terminal impedance setting circuit installed in a two-wire to four-wire (hereunder referred to as "2 to 4") converting circuit which is connected between a telephone exchange and a two-wire trunk line (or a central office line) leading to an opposite exchange system and which may have any of various kinds of characteristic impedance.
2. Description of the Prior Art
In a prior art 2 to 4 converting circuit, of the type described, a transformer is connectable to a two-wire trunk line which is provided with, for example, a 600 ohms/900 ohms switching terminal. The terminal allows either 600 ohms or 900 ohms to be selected by an operator in order to set up a terminal impedance which matches a particular characteristic of a trunk line to which the transformer is connected. In another prior art 2 to 4 converting circuit, a switch circuit is installed to automatically select either 600 ohms or 900 ohms in response to a control signal which is fed thereto from a central control circuit of an exchange system.
A reference is made to FIG. 1 for discussing such a prior art 2 to 4 converting circuit. In the drawing, a 2 to 4 converting circuit 80 connects a two-wire trunk line 50 to a four-wire analog/digital (A/D) converter 60, while matching the the impedance of the trunk to the impedance of the converter, i.e. 600 ohms or 900 ohms.
The converting circuit 80 is controlled by a central control circuit 70 of an exchange system. The converting circuit 80 includes a transformer 81, a switch 82, operational amplifiers 12 and 13, a resistor 100, and an echo canceller 83. The transformer 81 has one winding connecting to the trunk line 50. The other transformer winding is connected to the switch 82 at two terminals 811 and 812 thereof. The terminals 811 and 812 are respectively matched to 600 ohms and 900 ohms which are the characteristic impedance available with the trunk line 50.
In response to a command from the central control circuit 70, the switch 82 selects either one of the matching terminals 811 and 812 and connects it to the trunk line 50. In this instance, the difference of impedance value is dependent upon the number of turns of the transformer 81. The four-wire side of the switch 82 receives an output of the A/D converter 60 via the amplifier 12 and sends it to the trunk line 50. Further, the four-wire side of the switch 82 is connected to the input side of the A/D converter 60 via the amplifier 13 to deliver a signal which is coming in over the trunk line 50.
The switch 82 is implemented by an electromagnetic relay, for example. The connections are made to the terminals which are operated by a mechanical contact. The echo canceller 83 estimates an amount of feed back from the output of the amplifier 12 to the amplifier 13 and subtracts a signal component associated with the feed back from an output of the amplifier 13, in order to eliminate the feed back of speech.
A drawback with such a prior art construction is that one of the two windings of the transformer 81 has to be provided with an many as three terminals. This, coupled with the fact that the switch 82 is implemented by a mechanical relay, increases the overall size of the circuitry.
To eliminate the above described problem, switch 82 may be a semiconductor device. However, this kind of scheme is not practical for the following reason. A semiconductor switch generally has an imprecise internal conduction resistance value which is scattered over a substantial range of 50 ohms to 200 ohms. The resistance value of the resistor 100 usually lies in the range of 400 ohms to 600 ohms, which is a relatively low load impedance. Due to such a scattering of the conduction resistance of a semiconductor switch, the impedance produced by the resistor 100 and transformer 81 is not matched to the impedance of the trunk line 50.